Methods and implantable systems for neural sensing and nerve stimulation

ABSTRACT

Apparatus for producing a dorsiflexion of the foot of a patient comprises a combined sensing and stimulation electrode device having neurosense and stimulation electrode means capable of sensing a nerve signal from a peripheral nerve and stimulating peripheral motor nerve fibers. The neurosense and stimulation electrode means are implantable above the knee of a patient. Means for receiving and processing the sensed nerve signals are provided to identify a signal indicative of the timing of hell strike and heel lift of the patient and for producing a control signal in response thereto. Means for operating said neurosense and stimulation electrode means in response to the control signal to produce stimulation of the peripheral motor nerve fibers are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/203,908, filed Nov. 15, 2002, now U.S. Pat. No. 7,403,821which is the national stage of PCT/DK01/00112, filed Feb. 16, 2001,which in turn claims priority to Danish application no. PA 2000 00191filed Feb. 17, 2000, the entire contents of all of which areincorporated herein in their entirety by reference.

FIELD OF THE INVENTION

The present invention relates in its broadest concept to methods andsystems for electrically sensing signals originating from biologicaltissue and for the subsequent selective stimulation of muscular orneural tissue, the systems being at least partially implantable. Morespecifically, the present invention relates to the detection of neuralor muscular activity, analysis of the signals and the subsequentstimulating of neural or muscular tissue based thereon. In a specificaspect, the invention relates to correction of foot drop based on theseprinciples.

BACKGROUND OF THE INVENTION

Numerous conditions exist for the human body in which a disease orbodily malfunction have its origin in impaired function of theneuromuscular system, including deficiencies based upon impaired sensoryas well as motor activity. In the following the problems addressed bythe pre-sent invention will be discussed on the basis of the conditionknown as hemiplegic drop foot, a condition in which a patient is notable to lift, i.e. dorsiflex, the foot during gait, normally based on anUpper Motor Neurone Lesion (UMNL), for which stroke and head-injuriesare by far the more prevalent problems with reported prevelances of12,000/million for stroke and 20,000/million for head injuries.

Quite often persons who suffer a stroke recover a large amount offunction following a period of treatment, but a persistent, long-termdisability in approximately 10 to 20% of stroke survivors isUpperMotorNeurone-Drop Foot (UMN-DF). UMN-DF typically involves aninability to dorsiflex the foot during the swing phase of gait (DropFoot), as well as loss of normal hip and knee flexion, and inability to‘push-off’ as well as spasticity of the calf muscle group.

An important feature of UMNLs is that electrical excitability of theassociated peripheral nerves is still intact, thus facilitating the useof Functional Electrical Stimulation (FES) to restore or enhance gaitfor some of these cases. As early as in 1961, Liberson and hisco-workers proposed application of electrical stimulation (ES) to thecommon peroneal nerve to correct this condition and using a foot-switchsynchronised the application of ES to the swing phase of gait, using adevice subsequently referred to as a Peroneal Stimulator (PS) or DropFoot Stimulator (DFS) [Liberson, W. T., Holmquest, H. J., Scott, D. andDow, M. (1961) Functional Electrotherapy in stimulation of the peronealnerve synchronized with the swing phase of gait in hemiparetic patients.Arch Phys Med Rehabil 42, 202-205].

The development of FES-based Drop Foot correction has gone through thefollowing evolutionary stages:

-   (i) Hard-wired Single-channel Surface Drop Foot Stimulators,-   (ii) hard-wired Multi-channel Surface Drop Foot Stimulators,-   (iii) hard-wired Single-channel Implanted Drop Foot Stimulators,-   (iv) microprocessor-based Surface and Implanted Drop Foot    Stimulators,-   (v) alternative sensors as replacement for the Foot-switch: (a)    artificial sensors, and (b) “natural” sensors.

(i) Hard-wired Single-channel Surface DFS: As indicated above, the firstreported use of electrical stimulation for Hemiplegic Drop FootCorrection was in 1961 by Liberson who proposed to elicit dorsiflexionin a hemiplegic foot, synchronised with the swing phase of gait.Liberson's solution, shown in FIG. 1, comprised power and control box(1), a heel-switch (2), when open, during swing, open-circuits the shuntresistor (3), and enables the delivery of stimulus current across thestimulation electrodes (4). The switch when closed, during stance,connects the shunt resistor across the output of the stimulator and nostimulus is delivered to the stimulation electrodes. The delivery ofstimulus to the electrodes positioned for stimulation of the commonperoneal nerve) is therefore triggered when the heel-switch opens atreel-off and is terminated when the switch closes at heel-strike. Theapplication of stimulus is thus synchronised with the swing phase ofgait. This is an example of a hard-wired stimulator, where thefunctionality of the stimulator is determined by the wiring of theelectronic circuitry. The system performed the essential task ofinducing dorsiflexion in the subject's hemiplegic foot at theappropriate point in the gait cycle. Clearly, however, the functionalityof the system lacked sophistication and delivered stimuli in a crudefashion compared to the natural performance of the foot-lifterneuromuscular system.

(ii) Hard-wired Multi-channel Surface Drop Foot Stimulators: The firstgroup to present a major technical innovation on Liberson's design wasKralj and his co-workers from the University of Ljubljana in Slovenia.They proposed in 1971 the use of multiple channels of stimulation in thedrop foot stimulator and a radio link between the heel-switch and thestimulator [Kralj, A., Trnkoczy, A. and Acimovic, R. (1971) Improvementin Locomotion in Hemiplegic Patients with Multichannel ElectricalStimulation. In: Anonymous Human Locomotor Engineering—A review ofdevelopments in the field including advances in prosthetics and thedesign of aids and controls, pp. 45-50. Institute of Mechanicalengineers]. The proposed stimulator had three stimulation channelsenabling different muscle groups to be controlled independently, such asankle dorsiflexors and knee flexors and extensors.

(iii) Hard-wired Single-channel Implanted Drop Foot Stimulators: Thenext major development in hemiplegic drop foot correction technology wasthe investigation of the possibilities and practicalities of implantableHemiplegic Drop Foot Stimulators. Jeglic et al., proposed an implantedDFS (IDFS) aimed at overcoming problems of discomfort due to stimulationpain and difficulties experienced by subjects in correctly placing thestimulation electrodes [Jeglic, A., Vanken, E., Benedik, M.,“Implantable muscle/nerve stimulator as part of an electronic brace”, inProc. 3^(rd) International Symposium on External Control of HumanExtremities, 1970, pp. 593-603]. Jeglic et al's single channelstimulation device was never successful but may be seen as pre-datingthe development of a commercial, implantable, drop foot stimulator bythe Rancho Los Amigos Medical Centre/University of Southern Californiagroup in California, in conjunction with Medtronic Inc. of Minneapolis[Waters, R. L., McNeal, D. and Perry, J. (1975) Experimental Correctionof Foot-drop by Electrical Stimulation of the Peroneal Nerve. J. BoneJoint Surg [A] 57-A (8): 1047-1054].

The three elements of this system, shown in FIG. 2, are: an externalmodule with a transmitting antenna and control module (10), an implantedassembly comprising a receiver, pulse train generator and bipolarelectrode (11) and a heel-switch (12) located in the shoe. In commonwith the Jeglic et al. system, the implanted device required nobatteries as electrical power was supplied by electromagnetic induction.The antenna transmitted a radio-frequency signal through the skin andthis was taped to the skin directly over the implant. Two incisions wererequired: one on the medial aspect of the thigh to implant the receiver,another on the lateral aspect of the leg, below the knee, to expose thecommon peroneal nerve. The system was never put on the market,presumable due to the relatively extensive surgical procedures required.

(iv) Microprocessor-based Surface and Implanted Drop Foot Stimulators:The first use of micro-controller/microprocessor technology for DFSsystems is thought to be the incorporation of microprocessor technologyinto the previously discussed Multi-channel Surface Stimulators.

A primary motivation for the development of such multi-channelimplantable stimulators was to overcome the particular problem withsingle-channel implanted systems reported by Waters et al. [Waters, R.L., McNeal, D., Faloon, W. and Clifford, B. (1985) Functional ElectricalStimulation of the Peroneal Nerve for Hemiplegia—Long-Term Clinicalfollow-up. J. Bone Joint Surg [A] 67-A (5): 792-793]. Waters et al foundthat some of the subjects implanted with the Medtronic implantedsingle-channel DFS system walked with excessive inversion or eversionfollowing surgery. This problem was due to incorrect positioning of theelectrodes as the correct placement of the electrode is difficult todetermine during surgery. A balanced dorsiflexion response when thesubject is prone does not guarantee that the same response will beobtained when the subject is upright, weight-bearing and walking.

A solution to the problem of incorrect electrode placement duringsurgery, and of the tendency of the electrodes to move post-surgery wasproposed by Kelih et al. [Kelih, B., Rozman, J., Stanic, U. and KLjajic,M. (1988) Dual channel implantable stimulator. In: Walling a, W., Boom,H. B. K. and de Vries, J., (Eds.) Electrophysiological Kinesiology, pp.127-130. Elsevier Science Publichers B. V. (Biomedical Division)]. Theyproposed a dual-channel implantable stimulator enabling control oftwo-degrees of freedom of foot movement, viz. dorsiflexion and eversion.Thus, post-surgery, when the subject started to walk using the implant,the stimulus level on each channel could be adjusted to obtain balanceddorsiflexion.

(v) Alternative sensors as replacement for the Foot-switch: SinceLiberson's development of the first drop foot stimulator until the early1990s, the sensor used in FES-based Drop Foot Correction system had beenthe foot-switch, however, it has been proposed by several researchers,that it would be desirable to replace the foot-switch as the gait sensorin DFS systems for the following reasons:

(1) Fundamentally the traditionally foot-switch has been a contactsensor, requiring repetitive contact/non-contact of the wearer's footwith the foot-switch, which has major implications for the reliabilityof the sensor. With a DFS system, the ultimate application of the systemrequires that the subject brings the system home and wears it each day.For the wearer to accept this device and to overcome gadget intolerancethe reliability of the system must be high and failure of any componentof the system over a short period, including the gait sensor, isunacceptable.

(2) The accepted long-term approach to the implementation of FES-basedUMN-DF correction systems is the use of implanted systems. For acompletely implanted system, the ability to implant the gait sensor isdesirable and the foot-switch is unsuitable for implantation.

(3) Finally the information provided to the DFS system by a foot-switchis very limited, namely, presence or absence of contact by a part of thefoot with the ground. This type of signal is quite adequate for thehard-wired DFS systems described, but as the sophistication of DFSsystems is increased through the use of more complex control algorithms,the limitation of the foot-switch as a gait sensor should becomeapparent.

For the reasons outlined, several researchers have evaluated alternativegait sensors using either an artificial gait sensor which would besuitable for implantation or using the body's “natural” sensors.Developments in these two research areas will now be discussed.

(a) Artificial sensors as replacement for the Foot-switch: One of thefirst groups to propose alternatives to the foot-switch as a gait sensorin DFS systems was Symons et al. at the Rancho Los Amigos MedicalCentre/USC [J. Symons, D. McNeal, R. L. Waters, and J. Perry, “Triggerswitches for implantable gait stimulation,” in Proc. 9th Annual RESNAConference, 1986, pp. 319]. Symons carried out preliminary evaluation ofan in-house accelerometer fitted to the greater trochanter process ofthe femur in a vertical orientation to detect the heel strike event. Oneof the advantages of accelerometers is that they are miniaturisedintegrated electronic components and as such are highly reliable andtherefore very suitable candidates for implantation, which was therationale for the evaluation of the accelerometer.

Willemsen et al from the University of Twente in the Netherlands,proposed the used of an integrated accelerometer as a replacement forthe foot-switch in an UMN-DF correction system [Willemsen, A. T. M.,Bloemnof, F. and Boom, H. B. K. (1990) Automatic Stance-Swing PhaseDetection from Accelerometer Data for Peroneal Nerve Stimulation. IEEETransactions Biomedical Engineering 37 (12): 1201-1208]. In their paper,an arrangement of four commercial single-axis accelerometers was placedon the shank of a subject, as shown in FIG. 3. Willemsen et al was ableto distinguish between different phases of the gait cycle using theequivalent acceleration at the ankle joint as calculated from fouraccelerometers placed at locations 30 and 31 and was thus able to detectthe onset of swing (push-off) and the termination of swing(heel-strike). Careful attention was paid to the failure rate ofdetection of push-off and heel-strike. Out of a total of 106 steps,using three hemiplegic subjects, there were errors in only three steps,which is a very good performance.

U.S. Pat. No. 5,814,093 discloses the use of such an artificial sensorarranged corresponding to below the knee of a user. As has been thetradition until now, the (external) stimulation electrodes are placedbelow the knee joint.

(b) “Natural” sensors as replacement for the Foot-switch: A very elegantand powerful solution to the problems of gait sensors in FES-basedUMN-DF correction systems is to use the body's own sensing mechanism.Haugland and Sinkjaer described the use of recordings from a cuffelectrode, on the sural nerve, to control the application of stimulus tothe common peroneal nerve of a hemiplegic subject [Haugland, M. K. andSinkjaer, T. (1995) Cutaneous Whole Nerve Recordings Used for Correctionfor Foot-drop in Hemiplegic Man. IEEE Transactions BiomedicalEngineering 3 (4):307-317]. The device is shown in FIG. 4, and consistedof a power and control box 40, a set of surface electrodes 41 forstimulation of the peroneal nerve below the knee, a nerve cuff electrode42 implanted around the sural nerve 43, having percutaneous wires 44that with a connector 45 could be connected with the power and controlbox. To reduce noise in the nerve signal recording an external referenceelectrode 46 was strapped around the leg. The sural nerve is a sensorynerve which has as it sensory input touch sensors on the lateral part ofthe foot 47. It was proposed that the conventional heel switch in a DFSsystem be replaced by a single sural nerve cuff which monitored whetheror not the affected foot was supporting weight and used this informationto control the application of stimulus in the DFS. Recording nervesignals is referred to as Electroneurography and the correspondingsignal is called an ElectroNeuroGram (ENG). It was thus demonstratedthat nerve recordings could be used as the basis for the control of aDFS eliminating the need for an external foot-switch and its associatedproblems.

SUMMARY OF THE INVENTION

Having regard to the above discussion of the related prior art, thepresent invention has as its object to provide concepts, methods andimplantable systems which are more user-friendly than the systemshitherto proposed, including the system proposed by the presentinventors in 1995 and discussed above. More specifically, it is anobject of the invention to provide concepts, methods and implantablesystems which significantly

-   -   improves the reliability of the system,    -   increases the number of applications,    -   reduces the amount of surgery to be performed during        implantation and,    -   preferably, reduces the complexity of the implant.

In the present invention, one or more of the above objects are firstlyachieved by implantation of a stimulation electrode proximal to the knee(preferably a multi-channel electrode), for example on the commonperoneal nerve 5-10 cm proximal of the knee. This location provides anumber of advantages: more room for the stimulation device, lesssurgical intervention and better protection of the stimulation leadswhich do not have to cross the knee joint (as would be the case if theimplanted electronics were placed above the knee with the electrodesplaced below the knee at the conventional stimulation location),improved cosmetics, and thus patient acceptance, as a correspondingexternal unit can also be placed above the knee and thus more out ofsight. This aspect of the invention is based on the realisation thatcontrol of lower leg musculature can be achieved by carefullycontrolling the stimulation of a combined nerve, i.e. comprising a largenumber of motor and sensory fibres, located much more proximal thanhitherto considered possible.

Secondly, one or more of the above objects are achieved by a concept,method and system in which sensing and stimulation electrodes arearranged (or may be arranged) at a single location, located in such away that it serves as both a neural (or motor) sensing and neural (ormotor) stimulating means. This aspect of the invention is based on therealisation that single peripheral nerves with “useful” combinations ofsensing and motor fibres can be located and used purposefully. By theterm “single location” is meant that the electrodes are arranged in thevicinity of each other on a single peripheral nerve or a single spinalnerve root of a patient, the single location being accessible through asingle incision during surgical implantation.

In a preferred embodiment for the method of the invention, a specificuseful location is indicated for use in a HDP Correction System,however, the concept has a much broader approach as any kind of neuralsignal can be detected (also a natural motor-signal) resulting in theapplication of a stimulating neural impulse (which can also be appliedto a sensing nerve).

A first example of an alternative use of the present invention would beto provide a neural interface to an artificial prosthesis and based, forexample on the Median, Radial and/or Ulnar nerves in the arm or theSciatic nerve (or branches hereof, i.e. Tibial and Peroneal nerves) inthe leg. Efferent signals are recorded from one or more nerve stumps inthe amputated limb. These signals are then used for controlling themovement of the actions of the prosthesis. By proper transformation ofthe signals into actions of the prosthesis, control of the prosthesiscan become as natural as controlling a normal limb. Further, artificialsensors on the prosthesis can provide relevant feedback informationabout e.g. force and position, which is then relayed back to the user bystimulating the afferent fibres in the nerve stumps. In this way theuser can obtain sensation from the prosthesis that feel as if they camefrom the amputated limb.

A second example of an alternative use of the present invention would beto provide bladder control in patients with detrusor hyper-reflexia suchas spinal cord injured, multiple sclerosis and Parkinson patients basedon, for example, Sacral nerve roots. Implantable stimulators exist forcontrolling voiding in spinal cord injured subjects, however, thesesystems have no means for detecting when the bladder is full and thusneeds emptying. Further, they are incapable of detecting theinvoluntary, reflex mediated bladder contractions often experienced bythe same patients. These involuntary contractions often lead toincontinence and effectively a very small bladder capacity. To avoidthis the sensory parts of the nerve roots are often cut as part of theprocedure of implanting a void-controlling stimulator. With a combinedstimulating and recording electrode it would be possible to detectbladder fullness as well as the reflex contractions from these nerveroots. As it is possible to arrest bladder emptying by stimulation ofe.g. the pudendal nerve, the availability of information about the onsetof reflex contractions will make it possible to stop them. The method ofrecording natural nerve activity while at the same time being able tostimulate, might thus make it possible to avoid incontinence withoutcutting the sensory nerve roots, while at the same time being able toinform the user when the bladder is full.

A third example of an alternative use of the present invention would beapplied in a continent stoma. The stoma can be formed by e.g. divertinga muscle from its normal anatomical site and reconfiguring it as asphincter. Electrical nerve stimulation will be used to make the musclecontract and thus close the stoma aperture. As fatigue is a problem whenusing striated muscles for production of constant force over prolongedtime, it is important to reduce the stimulation to the minimallyrequired. Immediately after emptying the bowel, the stimulationintensity can be low. However, when the pressure in the bowel becomeslarger than the pressure produced by the stimulated muscle, the musclewill lengthen and an opening will start to occur that, if nothing isdone, will lead to incontinence. As the muscle used as the new sphinctercontains muscle spindles, it is possible to record the activity fromthese by means of the same electrode as is used for activating the motorfibres. The initial lengthening of the muscle can be detected from thespindle activity and be used for making the stimulation intensityincrease. In this way the sphincter is kept closed at all times with theminimum muscle force which reduces fatigue. Further, as the stimulationintensity will now monitor the pressure in the bowel, it is possible toprovide the system with an alarm, telling the user when it is time toempty the bowel.

A fourth example of an alternative use of the present invention would beto “amplify” a nerve signal to improve a muscle contraction or toimprove sensation. A weak nerve signal recorded from a nerve can beused, by proper transformation, for controlling a stimulus to the samenerve, thereby adding to the naturally occurring activity in the nerve.This can either be done by stimulating more nerve fibres than arealready active or by stimulating the already active nerve fibres with ahigher frequency (or both). In this way a too weak voluntary musclecontraction can be converted into a strong and functional movement. Thiscould e.g. be applied on the peroneal nerve in stroke patients who havea decreased (rather than completely missing) voluntary drive to the footdorsiflexor muscles or on the soleus or LGS nerves in the same patientsto increase the contraction of the ankle plantarflexor muscles toimprove the “push-off” phase of walking. Applied on sensory nerves,patients with reduced sensation can have the sensation increased from apart of the body.

A fifth example of an alternative use of the present invention would beto perform controlled decrease of activity in a nerve. A nerve cuffelectrode can be used for producing unidirectional antidromic actionpotentials in a nerve that by collision block stops naturally occurringactivity in the nerve. A method to control the level of blocking is tomonitor the activity passing the blocking electrode while the sameelectrode is used for production of the blocking potentials. In this waya closed-loop control algorithm can be provided for regulating the levelof stimulation so as to control the natural activity that is allowed topass by the electrode and continue to the innervated organ or muscle.One practical application of this could be to control the activity inthe autonomic nerves innervating the heart (the vagal cardiac branchesand the sympathetic cardiac nerves) under (cardio)pathologicalconditions.

A sixth example of an alternative use of the present invention would beto prevent involuntary spastic skeletal muscle contractions. Efferentsignals are recorded from one or more nerves innervating the spasticmuscles. When efferent nerve signals are recorded at times when the userwants the muscles to be relaxed, these nerve signals are used toactivate a unidirectional antidromic stimulation pattern that bycollision block of the nerve action potentials prevents the efferentnerve signals to reach the muscles and thereby prevent or stop a spasticmuscle contraction.

According to a first aspect of the present invention, there is providedapparatus for producing a dorsiflexion of the foot of a patientcomprising a combined sensing and stimulation electrode device havingneurosense and stimulation electrode means capable of sensing a nervesignal from a peripheral nerve and stimulating peripheral motor nervefibres, wherein said neurosense and stimulation electrode means areimplantable above the knee of a patient, means for receiving andprocessing the sensed nerve signals to identify a signal indicative ofthe timing of heel strike and heel lift of the patient and for producinga control signal in response thereto, and means for operating saidneurosense and stimulation electrode means in response to the controlsignal to produce stimulation of the peripheral motor nerve fibres.

In a preferred embodiment, the neurosense and stimulation electrodemeans comprise at least one combined sensing and stimulating electrodeconfigured for arrangement on a single peripheral nerve.

Preferably, the apparatus further comprises means for switching eachcombined sensing and stimulation electrode between a sensing state and astimulating state. In an embodiment, the means for switching eachcombined sensing and stimulating electrode between a sensing state and astimulating state comprises at least one opto-coupled field effecttransistor.

Alternatively and/or additionally, the neurosense and stimulationelectrode means comprise at least one neurosense electrode configuredfor arrangement on a peripheral nerve for sensing a nerve signal fromthe peripheral nerve, and at least one stimulation electrode configuredfor arrangement on a peripheral nerve for stimulating a peripheral motornerve fibre. In a preferred embodiment, the at least one neurosenseelectrode and the at least one stimulation electrode are configured forarrangement on the same peripheral nerve.

In an alternative embodiment, the at least one neurosense electrode andthe at least one stimulation electrode are configured for arrangement onadjacent peripheral nerves and are arranged to be located just next toeach other.

Preferably, the apparatus further comprises implantable communicationmeans for enabling communication between the operating means and acomputer located external to a patient's body, said communication meansbeing arranged to receive and/or store programs for use by the operatingmeans.

In a preferred embodiment, the receiving and processing means comprisefirst and second receiving and processing means, wherein the firstreceiving and processing means are arranged to process a signal receivedfrom the neurosense and stimulation electrode means and to provide afirst serially coded signal for transmitting to said second receivingand processing means, the second receiving and processing means comprisemeans for receiving the first serially coded signal, identifying thesignal indicative of the timing of heel strike and heel lift, providinga second serially coded control signal in response thereto andtransmitting the second) serially coded control signal to the operatingmeans, and the operating means are configured to decode the secondserially coded control signal and to provide a control signal fortransmission to the neurosense and stimulation electrode means.

Preferably, the first receiving and processing means and the operatingmeans are housed in the same device. More preferably, the device housingthe first receiving and processing means and the operating means isimplantable above the knee of a patient. More preferably, the devicehousing the first receiving and processing means and the operating meansis implantable in the thigh of a patient.

In an embodiment, the apparatus further comprises a power source forpowering the first receiving and processing means and the operatingmeans, wherein the power source is a rechargeable battery located in thedevice housing the receiving and processing means and the operatingmeans. Preferably, the rechargeable battery is inductively chargeable bya second power source located external to a patient's body.

Alternatively and/or additionally, the apparatus further comprises apower source for powering the first receiving and processing means andthe operating means, wherein the power source is located external to apatient's body. The apparatus may further comprise a power source forpowering the operating means, wherein the power source is implantablewith the operating means.

In a preferred embodiment, signals between the first receiving andprocessing means and the second receiving and processing means aretransmitted cordlessly through a patient's skin.

In a preferred embodiment, one or more of said sensing, stimulating andcombined sensing and stimulating electrodes are located in a single cuffand the cuff is configured for arrangement around a single peripheralnerve. Preferably, the peripheral nerve is the common peroneal nerve.Alternatively, the peripheral nerve may be the sciatic nerve.

In an alternative embodiment, one or more of said sensing electrodes arelocated in a first cuff and the first cuff is configured for arrangementaround a first single peripheral nerve and one or more of saidstimulating electrodes are located in a second cuff and the second cuffis configured for arrangement around a second single peripheral nerveadjacent the first peripheral nerve.

Preferably, the first peripheral nerve is the tibial nerve and thesecond peripheral nerve is the common peroneal nerve.

In a preferred embodiment, said neurosense and stimulation electrodemeans are implanted above the knee of a patient. In an alternativeembodiment, said neurosense and stimulation electrode means, firstreceiving and processing means and operating means are implanted abovethe knee of a patient.

In an alternative, embodiment, said neurosense and stimulation electrodemeans are implanted above the knee of a patient and said first receivingand processing means and operating means are implanted in the thigh of apatient.

It is to be noticed that the at least one neurosense electrode means andthe at least one stimulation electrode means are not necessarilyprovided as separate electrodes by may be incorporated in a combinedelectrode, further means being provided for switching each such combinedbetween a neurosense electrode state and a stimulation electrode state.In this way the most “compact” implementation of an apparatus accordingto the invention would comprise a single combined electrode incombination with corresponding switching means.

As will be apparent from the above disclosure of the prior art, it isoften desirable to provide a plurality of electrodes in a multichannelset-up, this providing enhanced control of both the sensing andstimulation aspects of the given implementation. However, the morechannels the more conductors will have to be used to transmit signalsbetween the electrodes and the processing means, this adding to the bulkof the implanted device as well as to the fragility. Accordingly, in apreferred embodiment of the invention, means are provided allowing areduced number of conductors to be used as compared to the number ofelectrodes, preferably by transmitting the signals serially coded andproviding the corresponding means for coding and de-coding the signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will hereinafter be described, withreference to the accompanying drawings, in which:

FIG. 1 shows schematically Liberson's solution for a hard-wiredSingle-channel Surface DFS;

FIG. 2 shows schematically the Rancho Los Amigos Medical Centrehard-wired Single-channel Implanted Drop Foot Stimulator;

FIG. 3 shows schematically Willemsen's set-up for an artificial sensoras replacement for the Foot-switch,

FIG. 4 shows schematically Haugland and Sinkjaers set-up for a naturalsensor as replacement for the Foot-switch,

FIG. 5 shows a diagram of drop foot system with implantable nervestimulator,

FIG. 6 shows a posterior view of the right popliteal fossa, the locationfor stimulation electrode on common peroneal nerve being indicated by aring,

FIG. 7 shows an example of torques measured around the ankle joint froma human subject, when stimulating the different sets of electrodes in a12-polar nerve cuff electrode implanted on the common peroneal nerveproximal to the knee,

FIG. 8 shows a diagram for a drop foot system with implanted nervestimulator and nerve signal amplifier for natural sensory feedback,

FIG. 9 shows a diagram of drop foot system with combined stimulation andrecording electrode implanted on the peroneal nerve,

FIG. 10 shows a nerve cuff placed on the sural nerve proximal to theankle and an implantable amplifier placed subcutaneously at the middleof the thigh,

FIG. 11 shows a diagram of a circuit making it possible to record andstimulate through the same cuff electrode,

FIG. 12 shows a raw signal recorded during stimulation,

FIG. 13 shows data recorded from the peroneal nerve cuff during gait,

FIG. 14 shows results from stimulating and recording at the same timefrom the same cuff,

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A first embodiment in accordance with a first aspect of the inventionwill be described with reference to FIG. 5-7. As can be seen from FIG.5, the system comprises an implantable multichannel stimulator with amultipolar nerve stimulation electrode implantable on the peronealnerve. Further it has an external unit, that powers and controls theimplant based on an external switch placed under the heel of the user.During gait, the heel-switch makes the stimulation turn an during theswing phase of the affected leg and off during the stance phase, to makethe dorsiflexor muscles actively lift the foot clear of the groundduring swing and to relax the dorsiflexor muscles during stance.

Peroneal Nerve (Implant Site for Stimulation Electrode)

In FIG. 6 is shown a schematic posterior view of the popliteal fossa inthe right leg, showing the thigh 60, knee 61, calf 62, Sciatic nerve 63,Tibial nerve 64, cutaneous branch of the Common Peroneal nerve 65, theCommon Peroneal nerve 66, the Sural Communicating nerve 67 and thepreferred location for the stimulation electrode 68. The stimulationelectrode is preferably implanted around the common peroneal nerve (CP),5-10 cm proximal to the knee, excluding the cutaneous nerves that branchoff the CP at this level. In this location the electrode is protected bythe two groups of tendons bordering the popliteal space. Further, whileit includes only few cutaneous fibres, it includes nerve fibres topractically all the muscles that dorsiflex the foot, i.e. muscles which:invert and dorsiflex the foot (tibialis anterior and extensor hallucislongus); dorsiflex and invert the foot (extensor digitorum longus andperoneus tertius); and evert and plantarflex the foot (peroneus longus)

This location of the electrode further has the advantage over locationsbelow the knee, (which have hitherto been used) that the lead-wires canbe passed to a suitable location for the main body of the implant, e.g.in the upper thigh or in the abdomen, without having to cross the kneejoint. Hereby the risk of lead breakage is reduced.

The stimulation electrode can be of many different designs. The commonfeature is that it should have several electrical contacts placed eitherinside or in the immediate vicinity of the nerve to be stimulated. Thecontacts should be placed so that when passing an electrical currentthrough them, different parts of the nerve will be activated. In apreferred embodiment, the electrode is an insulating structure thatcomprises a number of electrode contacts that provide an electricalinterface to the nerve. The contacts should be placed so that whenpassing an electrical current through them, different parts of the nervewill be activated.

The electrode means consists of one or more electrodes and a mechanicaldevice supporting their fixation onto the nerve. In a preferredembodiment, a nerve cuff electrode, which in principle is a tube of aninsulating material (such as silicone) with a number (possibly 12) ofmetal contacts (such as platinum, platinum-iridium or stainless steel)located within the cuff wall. The tube is opened in one side to make itpossible to install around the nerve, and a locking mechanism is used toclose the cuff hereafter.

In a preferred embodiment a silicone cuff with 12 platinum electrodecontacts, arranged as four tri-poles inside the cuff, is used. Eachtri-pole having the outer contacts short-circuited with a wire locatedinside the cuff wall, resulting in only eight wires exiting the cuff.The cuff is in a preferred embodiment ca. 20 mm long and has an innerdiameter approximately 20% larger than the nerve (typically 5.2 mm). Intests, this type of electrode has shown to provide enough selectivity inthe stimulation to have adequate control of foot movement for thepurpose of rehabilitation of foot drop.

In a first embodiment a multichannel stimulator is located in a separatehousing and connected to the stimulation electrode by means of amulti-lead cable. The cable has an in-line connector that can beassembled during surgery, to facilitate the passing of leads under theskin. The stimulator may have the following approximate dimensions:thickness=6 mm, length=60 mm, width=30 mm. In a preferred embodiment,the stimulator is implanted in the upper thigh of a patient. Thus, anexternal component of the device that communicates with the stimulatorcan be strapped to the upper thigh of the patient or held in a pocket ofan item of clothing worn by the patient. The purpose of the stimulatoris to deliver electrical pulses to the contacts in the stimulationelectrode. Usually these pulses will be charge-balanced andcurrent-controlled pulses, such as to minimise electrochemical processesat the electrode site and hereby reducing the risk of damage ofelectrodes and surrounding tissue. The stimulator is powered andcontrolled via an inductive link across the skin. The stimulator can beimplemented in various ways. The stimulator may be analog, havingseparate channels that are each powered and controlled with a separateinductive link, running at its own frequency (in a two channel systemthis could be 1.1 MHz and 4.4 MHz resp.). The device has been implantedin a small number of patients and has been used on a daily basis bythese patients. This stimulator produces fixed-current, biphasic pulsesthat are pulse width modulated by the external transmitter.

Another implementation of the stimulator could be by means of digitalelectronics. This method would be much to prefer, as it makes itpossible to establish a serial communication channel via the inductivelink and in this way program many more parameters of the stimulation.Furthermore, only a single inductive link will be necessary to supportany number of channels. A problem remains with the implant system asdescribed above. Each contact in the electrode requires a separateconductor in the cable (unless some contacts are connected directly inthe electrode). If it is wished to use many channels of stimulation,then the cable between stimulator and electrode becomes very thick,which is very impractical and more prone to be damaged by movement aswell as increasing the risk of damage to the nerve on which theelectrode is placed.

A solution to this is to place a part of the stimulator directly on theelectrode. It could then be made so that the cable only has to carrypower and serially coded information from the main body to theelectrode. At the electrode, this information is then decoded andstimulation pulses distributed to one or more of the contacts. In thisway the number of conductors in the cable can be reduced to somewherebetween two and four. This solution requires the electrode-part of thecircuit to be extremely miniaturised; otherwise it will not fit on theelectrode. A way to do this is by implementing it as a custom madeintegrated circuit. The above aspect of serially coding information alsoapplies to embodiments in which a plurality of channels is used for bothsensing and stimulating as will be described below. In such anembodiment the implanted electrode-part should compromise means forserially coding the sensed signals, for subsequent transmission to theprocessing means, and corresponding means for de-coding serially codedsignals transmitted from the stimulator.

On the surface of the skin over an implanted portion of the device,coil(s) for transmitting power and control information to the implantmay be placed. The coils are connected to a device with batteries,control circuitry and a power amplifier for powering the inductive link.

This unit can be designed in various ways. In a specific embodiment ithas been implemented with a micro-controller, that stores a program forcommunicating with the implant and that stores the parameters for thestimulation. The unit has inputs for power from a battery, signal from aheel-switch and a serial input channel for programming stimulationparameters into the unit. It further has a connector for downloading anew program into the unit.

In an alternative embodiment, power is supplied to the stimulator by achargeable battery located in the implanted stimulator. The rechargeablebattery may, for example, be a lithium-ion battery that may beinductively charged by an external charger.

A radio receiver is optional, but facilitates the programming ofparameters into the device from the programming unit and makes itpossible to have a wireless heel switch, as will be described below. Areceiver built by the present inventors has been based on one of theminiature transceiver circuits that are available commercially now (e.g.from RF Monolithics, Inc.). It provides a digital signal to the serialinput on the power and stimulus control unit and simply replaces astandard RS-232 cable.

In order to set up the system for a specific patient, a programming unitis necessary. This can be either a handheld dedicated unit, or a PC witha suitable program. If the power and stimulus control unit is equippedwith a radio receiver, the programming unit should have a correspondingtransmitter, to make it possible to transfer stimulus parameters withoutwires to the power and stimulus control unit.

This programming unit makes it possible to set the following parametersfor the stimulation:

-   -   threshold and maximum stimulation for each channel    -   stimulation frequency for each channel    -   the time allowed for ramping up the stimulation when the heel is        lifted from the ground for each channel

Including a ramped on/off is important for many of the patients, assimply turning on and off the stimulation abruptly will often cause astretch reflex to be initiated in the ankle plantar- anddorsiflexor-muscles respectively. The reflex is caused by too fastmovement of the joint and will always work against the desired movement.Choosing a suitable ramp speed for each of the activated channels caneliminate this reflex.

In a present embodiment the stimulation intensity is kept constantduring the swing phase of the leg (after ramping up to the specifiedlevel), however, in future embodiments it may prove beneficial to varythe intensity during the swing phase as well as for a brief period afterfoot-to-floor contact. Potential fatigue and discomfort of thestimulation may be reduced if the stimulation intensity is decreasedduring part of the swing phase. Further, it may be beneficial toincrease the stimulation intensity for a brief period after the heelcontacts the floor, to counteract the mechanical effects of the reactionforce from the floor acting upwards on the heel, which can cause thefront part of the foot to come down faster than preferred by the user.

Apart from the “advanced” programming unit, the patient nay be providedwith a box, that in a simple way (e.g. with a turnable knob or slider)can set the stimulation intensity and/or the balance between inversionand eversion, within a limited range set by the therapist. This userinterface may well set many or all of the stimulation parameters, butthe user should have only one or two knobs to set. An intelligentroutine in the programming unit then sets the specific parameters basedon the users command.

Heel Switch with/without Radio Transmitter

In the first preferred embodiment, the stimulation is controlled by asensor placed in the shoe of the user. This sensor can be a standardheel switch connected to the power and stimulus control unit with acable, or it can be different kind of sensor giving information aboutfoot-to-floor contact. Further, the cable can be replaced with awireless communication link.

In this first preferred embodiment the switch means has been implementedas a wearable garment that is mounted on the ankle of the user. Anultra-thin sensor (in this case a force sensitive resistor fromInterlink Electronics, Inc.) is sewn into the garment at a locationunder the heel. This sensor could also be made from a piezoelectricfilm, which has the advantage that it can provide power to parts or allof the electronic circuit in the device. On the garment above the ankleis placed a compact unit containing a battery, a transmitter (from RFMonolithics, Inc.) and a standard microcontroller. The microcontrolleris usually in a “sleep mode”, where it consumes only very little energy.When the user either steps on the sensor or releases the weight from it,the controller wakes up, transmits an identification number (to avoidinterference with other similar systems in the surroundings) and thenone of two possible codes (one for heel strike and one for heel lift) tothe power and stimulus control unit, and then goes back to sleep. Thecombination of a wearable garment with the device sewn in, the wirelesscommunication with the power and control box, and the fact that thesensor is very thin, makes it possible for the user to wear it all dayeven when changing clothes and footwear, to chose practically any typeof footwear and even walk without shoes.

Example 1

Three hemiplegic patients were equipped with the first embodiment of theinvention. It was tested if the system was able to produce dorsiflexionand if the multi-channel stimulation electrode placed on the peronealnerve above the knee was able to provide good selectivity so as tocontrol the different movements of the foot. With proper balancing ofthe different channels of stimulation, it was possible to produce amovement of the foot that was almost pure dorsiflexion, which is thenatural movement and thus most cosmetically acceptable and leastdamaging to the ankle during gait. In FIG. 7 an example of the torquesmeasured around the ankle joint of a human subject with a 12-polar nervecuff electrode implanted on the common peroneal nerve proximal to theknee. The foot was fixed during the measurements and the isometrictorques generated around the ankle were recorded while the stimulationintensity was increased from zero to maximum (pulse width 0-250 μs,current 1 mA, frequency 20 Hz) over a period of 5 seconds. It can beseen in FIG. 7 that the effect of the stimulation on the differentchannels were quite different, showing a good selectivity of the cuffelectrode and showing that the nerve fibres were functionally organisedin the nerve bundle. Channel A for low stimulation levels produced acombination of dorsiflexion with inversion, which at high stimulationlevels changed to eversion. Channel B produced a combination ofdorsiflexion and eversion. Channel C and D produced for low levels acombination of eversion and plantar flexion, which for higher levelschanged to dorsiflexion. During gait, relatively low torques arenecessary to produce movement, and by selecting a combination ofintensities on the different channels, it was possible to produce anatural dorsiflexion of the foot that felt comfortable for the patients(in the presented case by combining channel 1 and 2).

A second embodiment in accordance with the first aspect of the inventionwill now be described with reference to FIG. 8. Compared to the systemshown in FIG. 5, in this system the heel switch has been replaced with asystem for using the natural sensory nerve activity recorded from aperipheral nerve going to the foot. As many of the parts for the secondembodiment are the same as for the first embodiment, only the parts inFIG. 8 that are different from them in FIG. 5 will be described in thefollowing.

Sensory Nerve from the Foot

Neural information about foot-to-floor contact can be found in severalof the nerves innervating the skin of the foot. It may be expected thatthe best source of such signals will be the nerves innervating theplantar surface. However, the present inventors have found that alsonerves innervating other areas of the foot provide enough informationabout this that it can be extracted and used for control of thestimulator.

Signals from the calcaneal branch of the tibial nerve (innervating theskin on the heel) have been recorded, by implanting a nerve cuffelectrode on it, just proximal to the ankle joint [Upshaw and Sinkjaer(1998) Digital signal processing algorithms for the detection ofafferent nerve activity recorded from cuff electrodes, IEEE Trans.Rehabilitation Engineering, vol. 6, no. 2, 172-181]. This gives a goodsignal, but the surgical access to the nerve is too difficult for thisnerve to be a good candidate for a commercial system.

The sural nerve is a better candidate, as the surgical access to it ismuch easier. It also gives a signal that is as good as that recordedfrom the calcaneal nerve. Both signals clearly modulate with the gaitcycle and it is possible to detect heel strike and in most cases alsoheel lift from these signals. The sural nerve can easily be accessed afew centimeters proximal to the lateral malleol, where it is locatedimmediately under the skin.

For surgical reasons it is even better if the signals can be recordedmuch more proximal, preferably proximal to the knee. This may bepossible, as a branch of the common peroneal nerve communicates with thesural nerve (sural communicating branch (67)). This branch can be foundjust next to where the stimulation electrode is implanted which meansthat the recording electrode can be implanted through the same surgicalincision.

Nerve Recording Electrode

The nerve-recording electrode can be designed in various ways. Thecommon feature is that it will have one or more electrical contactsplaced in or around the nerve, so that the naturally occurring activityherein can be recorded, either as a sum of the activity from all thenerve fibres, a sum of the activity from a portion of the nerve fibresor the activity from a single nerve fibre.

In a preferred embodiment a nerve cuff electrode has been chosen, whichin principle is a tube of an insulating material (such as silicone) witha number of metal contacts placed on the inside of the tube. The tube,is opened in one side to make it possible to install around the nerve,and a locking mechanism is used to close the cuff hereafter. Our nervecuff has three internal circumferential electrodes (made of platinum orstainless steel) placed symmetrically in the cuff the at the centre andone at either end). It also has an external electrode, placed on theoutside of the cuff, which is used as a reference. The method forrecording nerve activity with such electrode has been described in theliterature. It provides a signal that is a weighted sum of the activityin all the nerve fibres inside the cuff and gives good rejection ofnoise signals external to the cuff. It has been shown to give stablesignals over long periods of time in chronic implants both in animalsand humans.

Implanted Amplifier and Telemeter

The purpose of this unit is to amplify the nerve signal recorded withthe recording electrode and transmit it through the skin to the externalnerve signal receiver. It can be implemented in various ways, dependingon the type of recording electrode. It should amplify the signal fromthe recording electrode with a suitable amount and optionally do somesignal conditioning (such as band-pass filtering, rectification and/orintegration) before transmitting the signal through the skin.

In a specific embodiment this has been implemented with a standardinstrumentation amplifier, that amplifies the signal with approximately120 dB and transmits the signal via an inductive link by frequencymodulation of the carrier signal. It has a bandwidth of 10 KHz. It ispowered via a separate inductive link from the external powertransmitter and signal receiver. The two inductive links transmitsignals through the same space and are uncoupled by a technique known asmorphognostic coils. Briefly this technique is based on shaping thecoils so that the summed magnetic flux from one inductive link goingthrough the coils of the other is zero. This is done by making the coilsof the power link be shaped as a square and the coils of the signal linkshaped as a figure eight. Further the two links run at separatefrequencies.

Power Transmitter and Signal Receiver

This unit forms the external part for the implanted amplifier andtelemeter. It provides an oscillating magnetic field from which theimplant gets its power and it receives the magnetic field from theimplant, that carries the amplified nerve signal. This latter magneticfield is picked up by a coil that matches the implanted transmittercoil, and the resulting electrical signal is demodulated to extract thenerve signal.

Signal Analysis

The purpose of the signal analysis unit is to transfer the recordednerve signal into a signal that replicates that of a heel switch if suchhad been placed in the shoe of the user. This can be implemented invarious ways. The most successful one to date is the following.

Pre-processing: The nerve signal is band-pass filtered to removeartefacts originating from ElectroMyoGraphic signals (EMG) from nearbymuscles and to reduce the content of electrode and amplifier noise. Thefilter cut-off frequencies are chosen based on an optimisation of thesignal to noise ratio using signals with known contents of signal andnoise. The signal is then rectified and integrated. To remove theartefacts coming from the stimulation pulses, the integration is done insections between the stimulation pulses, in such way that the integratedsections exclude the periods within which the stimulation artefactsoccur. This is a known technique called binintegration. The resultingsignal has a much lower bandwidth than the original nerve signal andrepresents the average amplitude of the nerve signal between eachstimulation pulse. This processing can be done either with analog ordigital circuitry. In any case the signal must come out of thepre-processing in a digitised format.

Detection of foot-to-floor contact: The digitised pre-processed nervesignal is fed into an adaptive logic network (ALN, available fromDendronic Decisions, Ltd.), which is a type of artificial neuralnetwork. This type of network is described in the literature, andconsists of a number of so-called linear threshold units and a number oflogic nodes. Due to the logic nature of the network it is very simple toimplement in a portable device, such as the micro-controller describedabove. The network takes as input a number of parameters derived fromthe pre-processed nerve signal and produces as an output a signal thatcan be used to turn on and off the stimulator. The network is trained bya supervised learning technique, that requires a sequence of data to berecorded from the patient during gait. During this sequence the patientis equipped with sensors under the foot. A set of data withcorresponding nerve signal and sensor-signal is presented to the ALN,and the training algorithm optimises the network so that based on thenerve signal it produces an output signal that matches the recordedsensor signal. Once this is done, the artificial sensors under the footcan be taken off and the ALN will control the stimulator based on thenerve signal alone.

If recordings are made from more than one nerve (e.g. the tibial and thesural or sural communicating branch), it will be possible to detectinformation about how the foot lands on the floor, e.g. if it lands onthe lateral or medial aspect of the foot sole, as different branches ofthe Tibial nerve innervate other areas of the foot-sole (lateral branch48, calcaneal branch 49 and medial branch 50, see FIG. 4) compared tothe sural nerve 47. This information can then be used for automaticregulation of the relative stimulation intensities to correct forexcessive inversion or eversion of the foot. Similarly, if it isdetected that the foot lands with the frontal part first, thenstimulation intensities can be increased so that better dorsiflexion isobtained to make to foot land with the heel first.

A third embodiment in accordance with a second aspect of the inventionwill now be described with reference to FIG. 9. This system combines thestimulation and recording electrode in one. This significantly reducesthe amount of surgery to be performed and reduces the complexity of theimplant. This can be done because the peroneal nerve contains cutaneous(sensory) fibres going to the foot, and it has proven possible to recordinformation from these sensors with the nerve cuff stimulation electrodedescribed above. The recorded signal modulates clearly with the gaitcycle and can be translated into a stimulator control signal in the sameway as described in the second preferred embodiment.

Compared to the systems shown in FIGS. 5 and 8, most of this thirdsystem is the same, except the sensory electrode and its cable aremissing and the amplifier and stimulator are built into the same device.This further results in the external equipment being simplified, as onlyone power transmitter is necessary.

Multipolar Nerve Stimulation and Recording Electrode

This electrode can be designed in various ways. However, the design mustfulfil the requirements for both recording and stimulation electrodes.The multipolar nerve cuff as described in the second embodiment issuitable for this.

Implanted Multichannel Stimulator and Nerve Signal Amplifier

This unit can be implemented in various ways. In principle it is only acombination of the stimulator and amplifier described with reference tothe first and second embodiments. However, it is necessary with a switcharrangement that can switch the electrode between being connected toeither the stimulator or the amplifier.

Preferably, this unit should contain the pre-processing as describedwith respect to the second embodiment above, as this greatly reduces thenecessary bandwidth for the implanted transmitter. If the stimulator isimplemented with digital electronics, it can be made able to storeinformation about the parameters of the pre-processing, to that this canbe adapted to the specific conditions of the particular patient, nerve,electrode etc. Also, it is then possible to send out the signal in adigital form, and thereby reduce the effect of loss of signal quality inthe transmission through the skin.

With the stimulator and the amplifier in this way connected, it is alsopossible to send other information from the implant to the external unitvia the same inductive link as transfers the amplitude of the nervesignal. This information could be e.g. impedance measurements of theelectrodes (to test the integrity of these), information about thehumidity within the encapsulation (to warn about potential malfunctionof the device), information about the coupling coefficient between theexternal and implanted coils (to facilitate placement of the externalunit), information about communication errors (handshaking) etc.

If selective recordings from the different sets of electrodes in theelectrode assembly are made, then it will be possible to detectinformation about how the foot lands on the floor, e.g. if it lands onthe lateral or medial aspect of the foot sole. This information can thenbe used for automatic regulation of the relative stimulation intensitiesto correct for excessive inversion or eversion of the foot. Similarly,if it is detected that the foot lands with the frontal part first, thenstimulation intensities can be increased so that better dorsiflexion isobtained to make to foot land with the heel first.

Example 2

As will be described in the following, the present inventors havedemonstrated that a cutaneous nerve signal from the foot can be recordedfrom the peroneal nerve proximal to the knee, using the same electrodeas was used for stimulation of the nerve to dorsiflex the foot. It wasalso demonstrated that it is possible to record this signal concurrentlywhile stimulating through the same cuff.

Methods

As part of the procedure for testing the implantable foot dropstimulator developed by the present inventors, three hemiplegic patientswere instrumented with a 12-polar nerve cuff electrode (20 mm long, 5.2mm ID) on the common peroneal nerve a few centimeters proximal to theknee. The eight lead-wires to the electrode were for an initial 8-weekperiod taken out through the skin. This procedure allowed directelectrical access to the cuff in this initial period.

One of the patients (female, 32 year old) further had a nerve cuffplaced on the sural nerve proximal to the ankle and an implantableamplifier placed subcutaneously at the middle of the thigh, see FIG. 10,which shows the Sciatic nerve 100, the Sural nerve 101, the Peronealnerve 102, the implanted amplifier 103 for amplification of the Suralnerve signal, the Sural nerve cuff electrode 104, the Peroneal nervecuff electrode 105 and the percutaneous wires 106 from the Peronealnerve cuff electrode.

To make it possible to record and stimulate through the same cuffelectrode, an electronic circuit was built, see FIG. 11, consisting of apersonal computer 110, a stimulator 111, opto-coupled field effecttransistors 112, the electrode 113, an amplifier 114 that transmittedthe amplified signal over a telemetric link 115 to a correspondingreceiver 116, a band-pass filter 117 filtering the signal to be between600 Hz to 2500 Hz. Controlled by an output from the stimulator 118, thecircuit switched the electrodes to the stimulator while a pulse wasissued and to the amplifier in the inter pulse intervals. The switcheswere made by opto-coupled field effect transistors, as these producedvery little switching noise (no charge injection through a gatecapacitance). Also, they had no galvanic connection between the switchedsignal and the control signal, which made them have very little effecton the common mode rejection properties of the amplifier. A moretraditional way of obtaining this would be use a transformer coupling,but transformers are inherently bulky and would be difficult to use inan implantable device. The stimulator produced charge-balanced, currentcontrolled pulses (1 mA, 0-255 μs pulse width). The PC controlled thestimulator and sampled and processed the amplified nerve signal.Clearly, the above-described embodiment is merely a laboratory testmodel, which for subsequent implementation would be miniaturized byincorporating the receiving and filtering means, the computing means aswell as the stimulator in a small and lightweight apparatus which can beeasily carried attached for example to the upper leg of a patient orcarried in a belt. Further, the stimulating signals and the power fordriving the electrodes 113 and thereto related circuitry would also besupplied to the implanted part of the system using a telemetric link asshown between the amplifier 114 and the receiver 116. Further, circuitrymay be associated with the electrodes 113 and the external components ofthe system for coding and de-coding the signals as described above.

FIG. 12 shows an example of the raw signal recorded during stimulation.The combined stimulation artefacts and switching noise were short enoughin duration (10 ms) to allow a sufficiently long window between them forrecording of the nerve signal.

FIG. 13 shows data recorded from the peroneal nerve cuff during gait(without stimulation). Top trace is the rectified and bin-integratedsural nerve signal 130, which is included for comparison only. Secondtrace is the corresponding peroneal nerve signal 131. For reference areshown the ankle angle 132 (recorded with a Penny & Giles goniometer) andsignals recorded from force sensitive resistors (FSR's) placed under thefoot at the heel 134, and at the medial 135 and lateral 133 metatarsals.

It is clear that the peroneal nerve signal modulated in synchrony withthe gait cycle and had features similar to the sural nerve signal. Itwas then investigated if the signal-processing algorithm for extractinga stimulator control signal could be applied on the peroneal nervesignal based on an adaptive logic network (ALN). This type of network isdescribed in the literature, and consists of a number of so-calledlinear threshold units and a number of logic nodes [A. Kostov, B. J.Andrews, D. B. Popovic, R. B. Stein, W. W. Armstrong (1995) Machinelearning in control of functional electrical stimulation systems forlocomotion IEEE Trans. Biomedical Engineering, vol. 42, no. 6, pp.541-551]. Due to the logic nature of the network it is very simple toimplement in a portable device, such as the micro-controller describedabove. The last trace 136 in FIG. 13 shows the result of this. Thenetwork was trained on 50 seconds of plain gait and evaluated on another50 seconds of plain gait. Comparing the target signal obtained from theFSR's with the output of the ALN yielded correct samples, which was ashigh as that obtained by using the sural nerve signal.

FIG. 14 shows results from stimulating and recording at the same timefrom the same cuff. The subject was sitting down. The circuit shown inFIG. 11 was connected to one set of electrodes in the peroneal cuff. Thefirst row 140 shows the stimulation intensity (pulse width inmicroseconds, current=1 mA, frequency=25 Hz). Second row 141 shows theoutput from a force sensor (un-calibrated) held manually on the dorsumof the foot to give indication of the effect of the stimulation. Thirdrow 142 shows the rectified and bin-integrated Peroneal nerve signal (inmicro Volts) Fourth row 143 shows a touch sensor (un-calibrated) thatwas used for manually stroking the skin to give an indication of whensensory signals should be expected. The data in the first column 144were recorded when was no stimulation and no touch of the skin. Therecorded signal thus represented the background activity of the system.In the second column 145 the foot was touched within the cutaneousinnervation area of the Peroneal nerve and the recorded nerve signalmodulated correspondingly. In the third column 146 stimulation wasapplied through the cuff but the skin was not touched. This gave a smallincrease in the nerve signal corresponding to the movement of the toescaused by the change in stimulation intensity but no apparent changethat could be related directly to the stimulation artefacts. In thefourth column 147 the Peroneal nerve was stimulated through the cuff andthe skin was touched. It can be seen that it was still possible to get aclear nerve signal from the nerve that modulated in accordance to thetouch of the skin, even when stimulating the same nerve with the sameelectrode.

CONCLUSIONS

It has been demonstrated that it is possible to stimulate a peripheralnerve and to record natural sensory information from this nerve via asingle set of electrodes, multiplexed fast enough to be functionallyconcurrent. This may be very useful in a number of applications wherenatural sensory feedback can add performance to a neuroprosthesis.

It has further been demonstrated that it is possible to recordgait-related information from the Peroneal nerve above the knee and useit to produce a control signal suitable for controlling a peronealstimulator for correction of foot drop.

1. Apparatus for producing a dorsiflexion of the foot of a patient,comprising: a combined sensing and stimulation electrode device havingneurosense and stimulation electrode means capable of sensing a nervesignal from a peripheral nerve and stimulating peripheral motor nervefibres, wherein said neurosense and stimulation electrode means areadapted to be implanted at a location on a single nerve above the kneeof the patient through a single surgical incision; means for receivingand processing the nerve signals sensed in from said location of thepatient above the knee of the patient to identify a signal indicative ofthe timing of heel strike and heel lift of the patient and for producinga control signal in response thereto, and means for operating saidneurosense and stimulation electrode means in response to the controlsignal to produce a stimulation of the peripheral motor nerve fibres atsaid location of to produce dorsiflexion of the foot of the patient. 2.Apparatus as claimed in claim 1, wherein the neuro sense and stimulationelectrode device comprise at least one combined sensing and stimulatingelectrode configured for arrangement on a single peripheral nerve. 3.Apparatus as claimed in claim 2, further comprising means for switchingeach combined sensing and stimulation electrode between a sensing stateand a stimulating state.
 4. Apparatus as claimed in claim 3, wherein themeans for switching each combined sensing and stimulating electrodebetween a sensing state and a stimulating state comprises at least oneopto-coupled field effect transistor.
 5. Apparatus as claimed in claim1, wherein the combined neurosense and stimulation electrode devicecomprises: at least one neurosense electrode configured for arrangementon a peripheral nerve for sensing a nerve signal from the peripheralnerve; and at least one stimulation electrode configured for arrangementon the same peripheral nerve for stimulating a peripheral motor nervefibre.
 6. Apparatus as claimed in claim 2 or claim 5, wherein one ormore of said sensing, stimulating and combined sensing and stimulatingelectrodes are located in a single cuff and the cuff is configured forarrangement around a single peripheral nerve.
 7. Apparatus as claimed inclaim 6, wherein the peripheral nerve is the common peroneal nerve. 8.Apparatus as claimed in claim 6, wherein the peripheral nerve is thesciatic nerve.
 9. Apparatus as claimed in claim 5, wherein one or moreof said sensing electrodes are located in a first cuff and the firstcuff is configured for arrangement around a first single peripheralnerve and one or more of said stimulating electrodes are located in asecond cuff and the second cuff is configured for arrangement around asecond single peripheral nerve adjacent the first peripheral nerve. 10.Apparatus as claimed in claim 9, wherein the first peripheral nerve isthe tibial nerve and the second peripheral nerve is the common peronealnerve.
 11. Apparatus as claimed in claim 5, wherein one or more of saidsensing, stimulating and combined sensing and stimulating electrodes arelocated in a single cuff and the cuff is configured for arrangementaround a single peripheral nerve.
 12. Apparatus as claimed in claim 1,further comprising implantable communication means for enablingcommunication between the operating means and a computer locatedexternal to a patient's body, said communication means being arranged toeither one or both of receive and store programs for use by theoperating means.
 13. Apparatus as claimed in claim 1, the receiving andprocessing means comprising first and second receiving and processingmeans, wherein: the first receiving and processing means are arranged toprocess a signal received from the neurosense and stimulation electrodemeans and to provide a first serially coded signal for transmitting tosaid second receiving and processing means; the second receiving andprocessing means comprise means for receiving the first serially codedsignal, identifying the signal indicative of the timing of heel strikeand heel lift, providing a second serially coded control signal inresponse thereto and transmitting the second serially coded controlsignal to the operating means; and the operating means are configured todecode the second serially coded control signal and to provide a controlsignal for transmission to the neurosense and stimulation electrodemeans.
 14. Apparatus as claimed in claim 13, wherein the first receivingand processing means and the operating means are housed in the samedevice.
 15. Apparatus as claimed in claim 14, wherein the device housingthe first receiving and processing means and the operating means isadapted to be implanted above the knee of a patient.
 16. Apparatus asclaimed in claim 14, wherein the device housing the first receiving andprocessing means and the operating means is adapted to be implanted inthe thigh of a patient.
 17. Apparatus as claimed in claim 14, furthercomprising a power source for powering the first receiving andprocessing means and the operating means, wherein the power source is arechargeable battery located in the device housing the receiving andprocessing means and the operating means.
 18. Apparatus as claimed inclaim 17, wherein the rechargeable battery is inductively chargeable bya second power source located external to a patient's body. 19.Apparatus as claimed in claim 14, further comprising a power source forpowering the first receiving and processing means and the operatingmeans, wherein the power source is located external to a patient's body.20. Apparatus as claimed in claim 13, wherein signals between the firstreceiving and processing means and the second receiving and processingmeans are transmitted cordlessly through a patient's skin.
 21. Apparatusas claimed in claim 1, further comprising a power source for poweringthe operating means, wherein the power source is adapted to be implantedwith the operating means.
 22. Apparatus as claimed in claim 1, whereinsaid neurosense and stimulation electrode means are adapted to beimplanted above the knee of a patient.
 23. Apparatus as claimed in claim13, wherein said neurosense and stimulation electrode means, firstreceiving and processing means and operating means are adapted to beimplanted above the knee of a patient.
 24. Apparatus as claimed in claim13, wherein said neurosense and stimulation electrode means are adaptedto be implanted above the knee of a patient and said first receiving andprocessing means and operating means are adapted to be implanted in thethigh of a patient.